Electron microscopy offers the opportunity to investigate the ultrastructure of a wide range of biological and inorganic specimens in 3-D at high resolution. In the field of biological sciences, for example, electron microscopy allows the observation of molecular mechanisms of diseases, the conformation of flexible protein structures and the behavior of individual viruses and proteins in their natural biological context. As another example, electron microscopy plays an important quality control function in the manufacture of semiconductors and electronic devices by allowing for the detection and characterization of nanoscale defects in electrical, optical, and micromechanical systems, which may affect the performance of such products. Defects can include contaminant particles that become embedded in a product during fabrication or a manufacturing defect, such as a bridge creating a short circuit between two closely spaced conductors that are intended to be electrically separated from each other. One technique employed with electron microscopy to carry out such investigations is called Slice-and-View™ (hereafter “slice and view”). This technique is typically performed with a dual beam system, that is, a system combining a focused ion beam (FIB) device and a scanning electron microscope (SEM) such as the DualBeam® instruments commercially available from FEI Company, the assignee of the present invention.
In the slice and view technique, as illustrated by FIG. 1, the focused ion beam cuts and slices a sample with high precision to reveal its 3D internal structures or features. Typically, the focused ion beam cuts exposes a cross section, or face, perpendicular to the top of the surface of the sample material having the hidden feature to be viewed. Because the SEM beam axis is typically at an acute angle relative to the focused ion beam cuts axis, a portion of the sample in front of the face is preferably removed so that the SEM beam can have access to image the face. After obtaining an image of the face by the SEM, another layer of substrate at the face may be removed using the focused ion beam cuts, revealing a new, deeper face and thus a deeper cross-section of the feature. Since only the portion of the feature at the very surface of the face is visible to the SEM, sequential repetition of cutting and imaging, or slicing and viewing, provides the data needed to reconstruct the sliced sample into a 3D representation of the feature. The 3D structure is then used to analyze the feature.
During slicing, variations in the topography of the surface exposed may occur as the focused ion beam traverses a sample. In some circumstances, the exposed face may have a surface morphology attributable to a phenomenon known colloquially as the “curtaining” effect, which is schematically illustrated in FIGS. 2A and 2B. When a sample 210 contains heterogeneous structures and/or compositions, the material removal rate of focused ion beam 215 from FIB column 220 may vary locally as focused ion beam 215 carries out a line mill 225 across face 230 in the direction of arrows 240. As a result, the surface of the sample exposed by the focused ion beam has a rippled face, or curtain. Local increases in the material removal rate may form concave curtains, such as curtaining artifacts 245A, 245B, and 245C, which penetrate into the face being exposed. Local decreases in the material removal rate may form convex curtains, such as curtaining artifact 250, which protrude from the face being exposed.
Software algorithms for 3-D reconstruction from slice and view imaging generally assume that surface of each slice imaged by the SEM is flat. Applicant has found that the presence of curtaining and other artifacts create topographical variations mean that the surface is not flat and this lack of flatness manifests as noise (e.g., decreased resolution) in the 3-D representation formed from images of the exposed surfaces. Thus, applicant has discovered that there is a need for methods, apparatuses, and systems that take into account variations in the topography of surfaces interrogated by an SEM during slice and view imaging of a sample, such as topographical variations caused by the curtaining effect, in order to improve the resolution of 3-D representations generated therefrom.